Polyurethane compositions have been proposed for and used in the preparation of seamless flooring for several years. In the year 1969, approximately 16 million gallons of clear urethane glaze were used for this purpose, indicating a dollar volume of about one hundred million dollars. In 1971, however, the sales were only about 2 to 5 million dollars, showing that the polyurethane glaze flooring as theretofore sold, did not meet expectations.
In the flooring as earlier used, a coating of moisture curing polyurethane (about 40% solids) in a solvent such as xylene or other solvent, was applied to the substrate to saturate it and provide adhesion. After the first coat had at least partially cured, a second coat was applied, and while the second coat was still wet, chips of colored dry paint were scattered over and pressed through the surface. When this layer had dried, the chips which had not adhered were swept away, the surface sanded and vacuumed, and another coat of clear urethane glaze was applied thereover. While that coat was still wet, chips were again scattered over the surface, pressed therein, and when the coating had hardened, the floor was again swept and vacuumed, and a further clear coat of urethane glaze (40% solids) applied. As soon as that coating had hardened, many more coats of 40% solids urethane glaze were applied thereover, each one after the former had hardened.
The flooring as thus produced had these drawbacks: The first coat did not adhere satisfactorily to grade level concrete. It was a 40% urethane glaze in hydrocarbon solvent and thus dissolved hydrocarbon soluble stains which were invariably present on the floor, and these stains migrated through to the top of the flooring. Furthermore, the urethane composition invariably turned brown after prolonged exposure to ultraviolet light. Finally the many coats required a great deal of labor, could not be applied in a single day, and the large quantities of solvent such as xylene caused considerable toxicity and odor.
Consequently, the industry evolved the following system:
1. A sealer coat was applied to keep out stains, give adhesion to any substrate, and bond to the next coat. This was either a 100%-solids epoxy or an epoxy emulsion.
2. A chip coat was developed to hold the chips, bridge cracks in the floor, and bond to the next coat. Ordinarily this was the same as the sealer coat.
3. A chip-binding coat or intermediate coat was applied to bind the very hydrophilic chips to each other and to harden them up enough so that they could be sanded. In most cases one or more coats of a polyurethane glaze was used for this purpose, but in some instances a polymeric latex or a clear epoxy emulsion was used. 4. Finally, glaze coats of curing polyurethane in solvent were applied to provide the wearing surface and to give abrasion resistance, stain resistance, and leveling. For each of the coats, an obvious requirement is rapid cure. Without it the job would take too long to be practical.
While the multi-layer system thus proposed is much superior to the original system, it still has many problems. When the epoxy emulsion or 100% solids epoxy is used for both sealer and chip coats, it becomes brittle, shrinks and cracks. This is permissible in a sealer coat, but not a chip coat, which must bridge cracks. In addition, the cure rate of the epoxies is very temperature dependent. Being two package materials, a material with a reasonably long pot life has an inordinately long cure time on a cold floor. Another problem peculiar to the epoxies, when used in urethane systems, is "purpling". The cause is not well understood, but in a significant number of cases the interface between the epoxy and an unpigmented urethane develops an unsightly purple color.
While the currently available base and chip coats present problems, they are more satisfactory than the currently available glazes which have the following faults:
A. The very high xylene content is unsatisfactory for two reasons -- the large amount of xylene is unpleasant and dangerous, and the solids content of the glaze is so low that several coats must be applied leading to high labor costs.
B. The glaze yellows badly because of the aromatic isocyanates used. Ultraviolet absorbers effectively halt yellowing only temporarily -- a few months to a year -- before the film yellows as much as if the absorber were not there.
C. The film formed from the glaze abrades rather quickly.
The obvious solution to the high xylene content, to use less reduction, did not work: Bubbles formed, which eventually broke and collected dirt. Apparently, reducing the xylene content permitted the surface skin to form. This stopped outward diffusion of carbon dioxide which, being entrapped, formed bubbles.
A solution to the yellowing problem is the use of the non-yellowing aliphatic isocyanates. All of these have certain disadvantages. For example, hexamethylene diisocyanate is extremely expensive, is highly toxic, and rather slow to cure. Hydrogenated MDI (HMDI) has two isocyanate groups with equal reactivity. Consequently, it forms highly viscous prepolymers which have a high percentage of free HMDI, which is extremely allergenic via skin absorption.
Isophorone diisocyanate, IPDI, a new material from Germany, seems to be the best isocyanate available. It is the lowest-priced aliphatic diisocyanate. Its two isocyanate groups are of unequal reactivity so that it gives lower-viscosity prepolymers containing less free monomer vis-a-vis HMDI. However, IPDI has serious drawbacks: although less toxic than HMDI or hexamethylene diisocyanate, it can still cause serious harm via skin absorption. Clear films from IPDI prepolymers degrade to liquid in strong sunlight. Prepolymers formed from the more reactive aliphatic isocyanate group of IPDI are terminated by the less-reactive cycloaliphatic isocyanate groups which moisture cures slowly.
The abrasion resistance problem is probably the most serious. Abrasion resistance is the property which is sought by the purchaser of a seamless floor. None of the solutions mentioned above helped to improve this property. The aliphatic isocyanates, rather than helping improve abrasion resistance, made it worse.
Floor surfaces, particularly those in public buildings, require not only abrasion resistance, but resistance to contamination or staining caused by tar or asphalt brought in by foot traffic from road or parking lot surface. To be a successful floor coating composition, the resulting coating must adhere strongly to the base, must dry or cure bubble free, must produce in a single application a heavy coating that is highly resistant to both abrasion and asphalt staining.
It is an object of the present invention to provide a laminated, seamless, polyurethane-base flooring that may be applied over a substrate such as concrete, wood, or the like, which adheres strongly, bridges small cracks, is not stained by migration of staining materials from the substrate, is stain resistant to asphalt, tars, and the like, has abrasion resistance superior to prior polyurethane-base floorings, is decorative, requires lower labor cost and less solvent than did urethane-base seamless floorings heretofore used, and which is free from the yellowing and deterioration caused by ultraviolet radiation.
It is another object of the present invention to provide a polyurethane-base coating composition of high solids content that may be applied over a suitably prepared base to provide a glaze layer of high wear resistance that resists staining by asphalt to a much greater extent than coatings previously proposed.
It is another object of the present invention to provide a polyurethane-base coating system that may be applied to suitably prepared vinyl tile, vinyl asbestos tile, wood, and the like. This will provide a clear, highly abrasion resistant coating which will not require expensive stripping and waxing.
It is a further object of the present invention to provide relatively low cost polyurethane-base coating compositions of high solids content which, although having excess of terminal --NCO groups, will have long term viscosity stability when maintained in moisture free containers, yet after it is mixed with a suitable crosslinking polyol, will set up and dry in a very few hours without bubbling.
Other objects of the present invention will become apparent from the following description of the invention.